Control device for power converter, control program and power conversion device

ABSTRACT

A control device includes a searching unit increasing or decreasing, at a predetermined step width, an operation voltage or an operation current of a power supply connected to a power converter to search a maximum power point of the power supply, a consecution time determining unit determining whether or not the searching unit has consecutively increased or decreased the operation voltage of the power supply or the operation current thereof, and a step width increasing unit increasing the step width upon determination by the consecution time determining unit that the increase or the decrease has been consecutively executed by a predetermined number of times.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims a priority of Japanese PatentApplication No. 2015-024295, filed on Feb. 10, 2015, the contents beingincorporated herein by reference.

FIELD

Embodiments of the present disclosure relate to a control device for apower converter which search the maximum power point of a power supply,a control program, and a power conversion device.

BACKGROUND

Dispersed power supplies, such as a solar light power generation device,a wind power generation device, a fuel cell battery, and a secondarybattery, need a power conversion device for a connection to a load andan interconnection with a system. For example, a solar power generationdevice is configured to be connected to a power conversion device calleda Power Conditioning System (PCS) in order to supply desired power to asystem.

Present PCSs include, in many cases, a power converter like a DC-DCconverter to boost up the voltage and a so-called inverter for DC-ACconversion, and a control unit thereof. In addition, the PCS includes aMaximum power point Tracker (MPPT) control unit that tracks the maximumpower point where the output power becomes the maximum during a changein amount of solar radiation time by time.

According to the MPPT, under the weather conditions, etc., that alwaysfluctuate, the value of current×voltage that maximizes the power, i.e.,the maximum power point is automatically obtained. According to such anMPPT, a generally adopted scheme to search the maximum power point is ahill climbing method. The hill climbing method is a method which changesthe battery voltage by a predetermined step width at a certain timeinterval, checks the increase or decrease of the output power, andincreases or decreases the voltage so as to always increase the outputpower, thereby searching the maximum power point.

As for the searching technologies relating to the hill climbing method,the following technologies are proposed.

(1) As the primary search, a searching operation is performed at a roughvoltage step width set within the certain voltage range. Next, after theneighborhood of the top among the plural mountains is detected, as thesecondary search, a searching operation is performed at a narrowervoltage step width than that of the primary search (see, for example,Patent Document 1).

(2) After the search within the certain voltage range, an approximatedcurve is calculated based on the relationship between the generatedpower and the voltage, and the voltage step width is set in accordancewith the curvature of the curved line (see, for example, Patent Document2).

(3) Across the maximum power point in the current-voltage curved line,the searching operation is divided into the current mode and the voltagemode. In the current mode, the current is changed at a predeterminedcurrent step width, and in the voltage mode, the voltage is changed at apredetermined voltage step width (see, for example, Patent Document 3).

(4) The detection error originating from capacitance parasitic in thesolar battery is reduced. That is, when the voltage and the current arechanged at fast speed, the current-voltage curved line is changed due tothe discharging and charging of the capacitance, generating a hysteresisloop. In order to eliminate this effect, the voltage and the current ata time point at which the voltage time-derivative value becomes zero aredetected (see, for example, Patent Document 4).

CITATION LIST Patent Literatures

Patent Document 1: JP 2012-221151A

Patent Document 2: Japan Patent No. 3359206

Patent Document 3: U.S. Pat. No. 8,754,627

Patent Document 4: Japan Patent No. 4491622

Meanwhile, according to the above technologies (1)-(4), there is a modeto search the maximum power point at a voltage step width within a givencertain range. Hence, searching operation requires time, resulting in aloss of power generation opportunity.

In addition, like the technology (4), according to the method ofobtaining a time derivative so as to reduce the effect of capacitanceparasitic in the solar battery, the detected value of the voltage, etc.,is unstable, and the derivative value is not likely to become zero.Hence, a determination should be made based on a fact that thederivative value becomes equal to or smaller than a certain threshold,but in this case, multiple control adjustments are necessary.

Still further, when the maximum power point is searched at fast speed,it is necessary to control the voltage of the capacitor or the currentof the reactor which is the energy buffer of the power conversion deviceat fast speed. In this case, the generated power of the solar batterychanges rapidly. Such a rapid change produces harmonic.

However, depending on a device and a system connected to the output ofthe power conversion device, since there is a limitation that does notaccept a rapid change, the fast-speed search is difficult in some cases.An example limitation is a harmonic restriction of an AC system to avoida radio disturbance therearound.

In order to address this technical problem, the power-voltagecharacteristic curve may be scanned at a constant cycle, and a powercontrol band may be extended. In order to do so, however, the costsincrease since the number of components increases and expensive controlcircuit and element are necessary.

Embodiments of the present disclosure have been proposed to address theabove technical problems of conventional technologies, and a firstobjective is to provide a control device for a power converter, acontrol program and a power conversion device which are capable ofsearching the maximum power point at fast speed in a generated powermaximizing control, decrease a loss of power generation opportunity toobtain large power, and need little control adjustment.

In addition, a second objective is to provide a control device for apower converter, a control program and a power conversion device whichare capable of reducing the size of an energy buffer and costs thereofwhile speeding up a search time.

SUMMARY

In order to accomplish the above objectives, a control device for apower converter according to an embodiment of the present disclosureincludes:

a searching unit increasing or decreasing, at a predetermined stepwidth, an operation voltage or an operation current of a power supplyconnected to the power converter to search a maximum power point of thepower supply;

a consecution time determining unit determining whether or not thesearching unit has consecutively increased or decreased the operationvoltage of the power supply or the operation current thereof; and

a step width increasing unit increasing the step width upondetermination by the consecution time determining unit that the increaseor the decrease has been consecutively executed by a predeterminednumber of times.

Note that as other aspects, the present disclosure is considerable as aprogram that causes a computer to accomplish the respective functions ofthe above units. In addition, as other aspects, also, a power conversiondevice including a power converter and the above control device is alsoaccomplishable.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a power conversion deviceaccording to an embodiment;

FIG. 2 is a diagram illustrating a voltage-power characteristic in anMPPT control by a general hill climbing method;

FIG. 3 is a diagram illustrating a voltage-power characteristic in anMPPT control according to a first embodiment;

FIG. 4 is a flowchart illustrating a process of increasing a step widthaccording to the first embodiment;

FIG. 5 is a block diagram illustrating an MPPT control unit according toa second embodiment;

FIG. 6 is a diagram illustrating a voltage-power characteristic in anMPPT control according to the second embodiment;

FIG. 7 is a flowchart illustrating a process of decreasing a step widthaccording to the second embodiment;

FIGS. 8(a)-8(c) are each an explanatory diagram for a resolution of acarrier wave, a capacity of a power converter, and a decrease width of avoltage, according to a third embodiment;

FIG. 9 is a block diagram illustrating an MPPT control unit according toa fourth embodiment;

FIG. 10 is a diagram illustrating a start position control according tothe fourth embodiment;

FIG. 11 is a block diagram illustrating an MPPT control unit accordingto a fifth embodiment;

FIG. 12 is an explanatory diagram for a slope limit according to thefifth embodiment;

FIG. 13 is a block diagram illustrating an MPPT control unit accordingto a sixth embodiment;

FIG. 14 is an explanatory diagram for a cycle of an AC voltage accordingto the sixth embodiment;

FIG. 15 is an explanatory diagram for a voltage instruction, a solarbattery voltage, and generated power when a search is performed at aconstant large step width;

FIG. 16 is an explanatory diagram for a voltage instruction, a solarbattery voltage, and generated power when a search is performed at aconstant small step width; and

FIG. 17 is an explanatory diagram for a voltage instruction, a solarbattery voltage, and generated power when a search is performed with astep width being reduced.

DETAILED DESCRIPTION First Embodiment

A power supply system 100 according to this embodiment will be explainedwith reference to FIG. 1. This power supply system 100 includes a powersupply 200 and a PCS 300.

[Configuration]

[Power Supply]

The power supply 200 is a source of supplying power. An example powersupply 200 is a dispersed power supply. The dispersed power supply is apower generation facility disposed and dispersed at a location nearerthe power demanding site in comparison with a large-scale powergeneration plant. As for the power supply 200 in this embodiment, asolar light power generation device, a wind power generation device,etc., which have relatively unstable generated power are suitable, butthe present disclosure is not limited to those examples. A fuel cellbattery, a secondary battery, etc., are also applicable. The followingexplanation will be given of an example case in which the power supply200 is a solar power generation device.

[PCS]

The PCS 300 is a power conversion device that converts the generatedpower by the power supply 200 into power appropriate for a load and asystem. The PCS 300 includes a power converter 400, and a control device500.

The power converter 400 is a circuit that converts input power. Thepower converter 400 includes, for example, a DC-DC converter that boostsup the DC voltage from the power supply 200, and an inverter thatconverts the DC power into AC power and outputs this AC power. The DC-DCconverter, and the inverter each include switching elements, and arecapable of outputting desired power by the switching operation. Theswitching element is a self-extinguishing type element, such as a GTO, aMOSFET, an IGBT, or an IEGT, and is connected to a power supply and adrive circuit.

The power converter 400 is not limited to the above configuration.Various converters available presently or in future are applicable. Forexample, the power converter 400 that includes only the DC-DC convertermay be applied to supply DC power. In addition, the power converter 400that includes only the inverter and has the DC-DC converter omitted isalso applicable. Still further, the power converter 400 may be a ModuleIntegrated Converter (MIC), such as a DC-DC optimizer or amicro-inverter, placed individually in not the PCS 300 but each powersupply 200 like the solar power generation device.

The control device 500 controls the power converter 400. The controldevice 500 is accomplished by a computer that is controlled by apredetermined program, or a special-purpose electronic circuit. In thiscase, the program physically utilizes the hardware resources of thecomputer to accomplish the process of each component to be explainedlater. Note that the method of executing the process by each component,the program, and a non-transitory recording medium having stored thereinthe program are also aspects of the embodiment. In addition, how to setthe range processed by the hardware resource, and the range processed bya software including the program is not limited to any particularmanner.

This control device 500 includes a memory 10, a converter control unit20, and an MPPT control unit 30. The memory 10 is a process unit thatstores various information necessary for a power conversion process bythe PCS 300. As for the memory 10, all memory media that are availablepresently or in future, such as a semiconductor memory and a hard disk,are applicable. A memory medium that has already stored thereininformation may be loaded in a reader device to utilize the storedcontents for various processes, thereby accomplishing the memory 10.

This memory 10 includes a main memory device that stores a program,etc., and a cache memory, a buffer memory, a register, etc., which areutilized as a temporal memory area. A memory area for information inputfrom the exterior via unillustrated sensor and network, and informationexchanged by buffering a difference in process timing among respectivecomponents are also considerable as the memory 10.

Example information stored in the memory 10 is information detected andinput by the sensor, information input from the exterior via a network,and information generated by the converter control unit 20 and the MPPTcontrol unit 30 to be explained later. Such information contains theoperation voltage of the power supply 200, the operation currentthereof, the generated power by the power supply 200, a voltageinstruction value, and a current instruction value, etc. The operationvoltage, the operation current, the generated power, the voltageinstruction value, and the current instruction value may be each adetected value, or may be an estimated value obtained by an arithmeticprocess of the control device 500 based on any actual measured value.

In addition, this information also contains a setting value that is setup beforehand for the process by the MPPT control unit 30 and theprocess by the converter control unit 20. The setting value contains asearch start position, the MPPT control timing, the step width for MPPT,a number of consecution times, the increase of step width, a number ofrepeat times, the decrease of step width, the resolution of a carrierwave, the minimum width of the step width, a search range covering theupper limit and the lower limit, a number of reaching times to the upperlimit or the lower limit, and the cycle of AC power. How to utilizethose pieces of information will be explained later.

The converter control unit 20 is a process unit that controls the powerconverter 400. For example, the converter control unit 20 outputs anON-OFF switching instruction for the switching elements of the powerconverter 400 to the drive circuit, thereby causing the power converter400 to output power in accordance with the voltage instruction value orthe current instruction value. The control by the converter control unit20 is performed by, for example, a PWM control. The PWM control is acontrol to turn ON the switching elements within a time period in whicha modulation wave is larger when compared with a carrier wave.

The MPPT control unit 30 is a process unit that performs a generatedpower maximizing control. That is, the MPPT control unit 30 searches theoperation point where the output power by the power supply 200 becomesthe maximum. This MPPT control unit 30 includes a searching unit 31, aconsecution time determining unit 32, and a step width increasing unit33.

The searching unit 31 is a process unit that increases or decreases theoperation voltage of the power supply 200 connected to the powerconverter 400 or the operation current thereof at a predetermined stepwidth, thereby searching the maximum power point of the power supply200. That is, the searching unit 31 performs an MPPT control by a hillclimbing method. According to the hill climbing method, for example, asindicated by the power-voltage characteristic illustrated in FIG. 2, theoperation voltage of the power supply 200 is changed, at a predeterminedcontrol timing, by ΔV that is the predetermined step width, therebychecking whether or not the output power is increased or decreased.

Next, Δv is increased or decreased in the increasing direction. Byrepeating this operation, a maximum power point Pmax is searched. Thepredetermined control timing is a certain time interval set in thememory 10 beforehand. The predetermined step width is a variabilitywidth of the voltage or the current set in the memory 10 beforehand.

Note that the searching unit 31 starts the searching operation from asearch start position set in the memory 10 beforehand. The search startposition is any position between the position corresponding to ashort-circuit current at zero voltage in FIG. 2 and the positioncorresponding to an open voltage at zero current. Hence, the search maystart from the short-circuit-current position or the open-voltageposition. As will be explained later, however, when a search range isset, the search starts from either the upper limit or the lower limit ofthis range.

The consecution time determining unit 32 is a process unit thatdetermines whether or not the searching unit 31 has consecutivelyincreased or decreased the operation voltage of the power supply 200 orthe operation current thereof by a predetermined number of times. Theterm consecutively increase or decrease means every time the search isexecuted at a constant control cycle, the operation voltage or theoperation current which has been increased is further increased or whichhas been decreased is further decreased as illustrated in FIG. 2 duringthe process of increasing or decreasing the operation voltage or theoperation current.

The step width increasing unit 33 is a process unit that increases thestep width when the consecution time determining unit 32 determines thatthe continuous increase or decrease has been performed by thepredetermined number of times. The predetermined number of times is anumber of consecution times set in the memory 10 beforehand. Anincrement value is a value to increase the step width and set in thememory 10 beforehand. As for the increment value, an addition value maybe set or a numerical value to be multiplied for the increase may beset.

When, for example, the predetermined number of times is two, asindicated by the power-voltage characteristic illustrated in FIG. 3, itis assumed that the searching unit 31 consecutively increases anoperation voltage V1 to V2, and to V3 twice by ΔV1, and power isincreased from P1 to P3. In this case, the step width increasing unit 33increases by ΔV2 that is the increase value to which ΔV1 is furtheradded.

In addition, although it is not illustrated in the figure, the PCS 300is connected to an input unit and an output unit. The input unit is acomponent to input necessary information for the process by the PCS 300,a selection of a process, and an instruction. Through input unit, anoperator is capable of inputting information to be stored in the memory10. Example input units are all input devices available presently or infuture, such as a keyboard, a mouse, a touch panel (including onedisplayed on a display device), a switch and a sound input device.

The output unit is a component that enables the operator to recognizethe information stored in the memory 10 of the PCS 300, the processresult of each unit, etc. Example output units are all output devicesavailable presently or in future, such as a display device, a printer, agauge, a lamp, a speaker, and a buzzer. For example, by displaying theinformation in the process on the display device in the form of anumeric number, a graph, etc., the operator is capable of checking theoperation status. Note that the input unit and the output unit alsoinclude a console panel of the PCS 300, a computer terminal for anoperation connected to the PCS 300 via a cable or a network.

[Action]

An action of this embodiment will be explained with reference to FIGS. 3and 4. In the following example, basically the same process is appliedwhen the operation voltage of the power converter 400 is changed toperform a power maximizing control and when the operation current ischanged to perform the power maximizing control.

(Process Outline)

First, as illustrated in FIG. 3, the MPPT control unit 30 of thisembodiment changes the voltage from a low voltage to a high voltage bythe constant step width Δv1 in the direction in which the powerincreases from P1 to P3. When the voltage is changed multiple times inthe same direction, this means that the search has not reached themaximum power point Pmax. In this case, the MPPT control unit 30increases the step width to Δv2 that is larger than Δv1, therebyperforming a control so as to reach the maximum power point Pmax faster.

(Process Procedure)

The procedure of the process of increasing the step width according tothis embodiment as explained above will be explained with reference tothe flowchart of FIG. 4. Note that this increasing process is a partextracted from a process consecutively executed during the powermaximizing process by the searching unit 31. That is, a process per anincrease of the step width will be explained.

At the start of the MPPT control by the searching unit 31, theconsecution time determining unit 32 initializes a variable N thatindicates the number of consecution times (step S01). Next, theconsecution time determining unit 32 determines (step S02) whether ornot the searching unit 31 has consecutively increased or decreased thevoltage value during the searching. When determining that the searchingunit 31 has consecutively increased or decreased the voltage value (stepS03: YES), the consecution time determining unit 32 increments (stepS04) the variable N that indicates the number of consecution times.

Next, the consecution time determining unit 32 determines (step S05)whether or not the variable N reaches a predetermined number of times Y.When determining that the variable N has not reached the predeterminednumber of times Y (step S06: NO), the consecution time determining unit32 returns the process to a consecution determining process (step S02).Next, when determining that the searching unit 31 has consecutivelyincreased or decreased the voltage value (step S03: YES), theconsecution time determining unit 32 increments (step S04) the variableN.

The consecution time determining unit 32 determines (step S05) whetheror not the variable N has reached the predetermined number of times Y.When the consecution time determining unit 32 determines that thevariable N has reached the predetermined number of times Y (step S06:YES), the step width increasing unit 33 increases (step S07) the stepwidth for the search by the searching unit 31.

Conversely, when the consecution time determining unit 32 determines(step S03: NO) that the voltage value has not been consecutivelyincreased or decreased, the variable N is initialized (step S01). Hence,only when the operation voltage is consecutively increased or decreased,the next step width increasing process is executed.

The searching unit 31 keeps searching at the increased step width, andwhen the voltage value is further consecutively increased or decreased,the above step width increasing process is further executed.

[Effects]

The control device 500 for the power converter 400 includes thesearching unit 31 that increases or decreases the operation voltage ofthe power supply 200 connected to the power converter 400 or theoperation current thereof by the predetermined step width to search themaximum power point of the power supply 200, the consecution timedetermining unit 32 that determines whether or not the searching unit 31has consecutively increased or decreased the operation voltage of thepower supply 200 or the operation current thereof, and the step widthincreasing unit 33 that increases the step width when the consecutiontime determining unit 32 determines that the operation voltage orcurrent has been consecutively increased or decreased by thepredetermined number of times.

When the searching unit 31 is performing the searching in the directionin which the voltage value is consecutively increased or decreased bythe predetermined number of times, this means that the search has notreached the point where the generated power becomes the maximum. In thisembodiment, the step width is increased while the voltage valueincreased by the predetermined number of times is detected. Hence, thesearch is capable of reaching the point where the generated powerbecomes the maximum faster, decreasing the loss of power generationopportunity, and increasing the amount of power to be obtained by powergeneration.

When the search start point by the searching unit 31 is set at aposition corresponding to the short-circuit current or the open voltage,the search stars from the far position away from the maximum powerpoint. In this case, the speed-up of the search by the increase ordecrease of the step width according to this embodiment is effective.

In addition, the search start point by the searching unit 31 may be setat a high-voltage side relative to the maximum power point. In thiscase, as illustrated in FIGS. 2 and 3, since the slope of the curvedline of the power-voltage characteristic is steeper than that of thelow-voltage side, the search is capable of reaching the maximum powerpoint further faster. For example, the search may start from theposition corresponding to the open voltage.

Second Embodiment

[Configuration]

This embodiment employs basically the same configuration as that of theabove first embodiment. However, as illustrated in FIG. 5, the MPPTcontrol unit 30 of this embodiment includes a repeat time determiningunit 34, and a step width decreasing unit 35. The repeat timedetermining unit 34 is a process unit that determines whether or not thesearching unit 31 has repeatedly increased or decreased the operationvoltage of the power supply 200 or the operation current thereof.

The step width decreasing unit 35 is a process unit that decreases thestep width when the repeat time determining unit 34 determines that therepeating has been executed by a predetermined number of times. Thepredetermined number of times is set in the memory 10 beforehand, and isan expected number of repeat times of the increase or the decreasearound the maximum power point. The decrement value is a value which isset in the memory 10 beforehand and which decreases the step width. Asfor the setting of decrement value, a decrease amount may be set, or anumerical value for a multiplication or division for decrease may beset.

[Action]

The action of this embodiment will be explained with reference to FIG. 6and FIG. 7. In the following explanation, basically the same process isapplied when the operation voltage of the power converter 400 is changedto perform the power maximizing control and when the operation currentis changed to perform the power maximizing control.

(Process Outline)

When the voltage step width is large, although the search time becomesshort, the generated power may temporarily decrease. Conversely, whenthe voltage step width is small, although the search time becomes long,the generated power does not decrease. In order to accomplish awell-balanced effect in this trade-off relationship, a case in which thesearch is far away from the point where the generated power becomes themaximum and a case in which the search is close to that point areestimated based on the action in accordance with the hill climbingmethod, and the voltage step is increased when the search is far, andthe voltage step is decreased when the search is close.

That is, according to this embodiment, basically the same operation asthat of the first embodiment is executed. However, as illustrated inFIG. 6, according to the search by the searching unit 31 through thehill climbing method, when the voltage of the power supply 200 comes toa point near the maximum power point, the increase and decrease of thevoltage value are repeated. Such increase and decrease will be referredto as a reciprocating operation.

In the reciprocating operation condition, the maximum power point ishighly possibly present within this range. Hence, by reaching themaximum power point with the step width being decreased, the generatedpower in the steady condition is stably increased.

(Process Procedure)

The procedure of the process of increasing the step width according tothis embodiment as explained above will be explained with reference tothe flowchart that is FIG. 7. Note that this process is a part extractedfrom a process consecutively executed in the power maximizing process bythe searching unit 31. That is, a process per a decrease of the stepwidth will be explained. In addition, this process is executed when thesearch by the searching unit 31 comes around the maximum power pointthrough the process of the above first embodiment.

The repeat time determining unit 34 initializes (step S11) a variable Rthat indicates the number of repeat times in the start of the MPPTcontrol by the searching unit 31. Next, the repeat time determining unit34 determines (step S12) whether or not the searching unit 31 hasdecreased or increased subsequent to the increase or decrease of thevoltage value during the search. When determining that the voltage valuehas been decreased or increased subsequent to the increase or decreasethereof (step S13: YES), the repeat time determining unit 34 increments(step S14) the variable R that indicates the number of repeat times.

Next, the step width decreasing unit 35 determines (step S15) whether ornot the variable R has reached a predetermined number of times Z. Whendetermining that the variable R has not reached the predetermined numberof times Z (step S16: NO), the step width decreasing unit 35 returns(step S12) the process to an increase and decrease determining process.

In addition, when determining that the searching unit 31 has increasedor decreased the voltage value (step S13: YES), the repeat timedetermining unit 34 increments (step S14) the variable R. The step widthdecreasing unit 35 determines (step S15) whether or not the variable Rhas reached the predetermined number of times Z. When determining thatthe variable R has reached the predetermined number of times Z (stepS16: YES), the step width decreasing unit 35 decreases (step S17) thestep width for the search by the searching unit 31.

Conversely, when determining, in the step S12, that the voltage valuehas not been increased and decreased (step S13: NO), the repeat timedetermining unit 34 initializes (step S11) the variable R. Hence, onlywhen the operation voltage is consecutively increased and decreased, thestep width decreasing process is executed.

The searching unit 31 keeps searching with the decreased step width, butevery time the increase and decrease are repeated, the above step widthdecreasing process is executed to search the maximum power point.

[Effects]

According to this embodiment, in addition to the configuration of theabove first embodiment, the searching unit 31 includes the repeat timedetermining unit 34 that determines whether or not the increase anddecrease of the operation voltage of the power supply 200 or theoperation current thereof are consecutively repeated by thepredetermined number of times, and the step width decreasing unit 35that decreases the step width when the repeat time determining unit 34determines that the increase and decrease are consecutively repeated bythe predetermined number of times.

Hence, first, like the first embodiment, when the operation voltage isconsecutively increased, it is estimatable that the search is still faraway from the maximum power point, the search time is reduced byincreasing the step width. In addition, when the increase of theoperation voltage and the decrease thereof are repeated, it isestimatable that the search is near the maximum power point, and thusthe reduction of the generated power is prevented by decreasing the stepwidth. Hence, in addition to the first embodiment, by combining theoperation of this embodiment, an effect of accomplishing both the searchat short times and the maximized power generation is obtainable.

Third Embodiment

This embodiment employs basically the same configuration as that of thesecond embodiment. However, the minimum value of the step width set inthe memory 10 beforehand is set based on the resolution of the carrierwave compared with a modulation wave by the converter control unit 20.That is, when a digital control is performed, as illustrated in FIG.8(a), the carrier wave has a minimum resolution n that is a controlcycle for a comparison with the modulation wave.

In this embodiment, like the second embodiment, this resolution n is setas the minimum step width when the step width is decreased. When, forexample, the power converter 400 is a voltage type, a change in voltageis defined as the resolution of the carrier wave to be compared with themodulation wave and when the power converter 400 is a current type, achange in current is defined as such a resolution.

FIG. 8(b) illustrates an example case in which the power converter 400is a voltage type. In general, the voltage-type power converter 400 hasa capacitor C that servers as an energy buffer which accumulates energyin the form of voltage. The voltage varies to some level depending onthe accumulated energy amount. When this variability range is defined asVmax−Vmin, as illustrated in FIG. 8(c), a value obtained by dividing theminimum value Vmin by the resolution n of the carrier wave is set to theminimum step width ΔV that is to be subtracted from the voltage valueVc.

Hence, in the voltage-power characteristic of the power supply 200, areduction of the generated power is suppressed, and a steadymaximization of power generation is enabled. Note that in the case of acurrent-type power converter 400 that has a reactor serving as an energybuffer, the minimum step width to decrease the current value may be setwith reference to the resolution. It is not always necessary that thestep width matches the minimum resolution. When, for example, theresolution is sufficiently high, the step width may be, for example,twice as much as the minimum resolution.

Fourth Embodiment

[Configuration]

This embodiment employs basically the same configuration as that of thesecond embodiment. However, as illustrated in FIG. 9, the MPPT controlunit 30 of this embodiment includes a start position instructing unit36, and a search terminating unit 37. In addition, the search range bythe searching unit 31 is set in the memory 10 beforehand. The searchrange is between the upper limit and the lower limit of the voltage orthe current to be searched.

The major reasons why such a search range is set are as follows:

(1) Strategic reason on whether the preference is given to a search timeor a precise search; and

(2) Restriction over the specification of the power converter 400.

As for the reason (1), first, the maximum search range is a rangeequivalent to the short-circuit current and the open voltage. Byadjusting this search range, adjustment of the search time is enabled.For example, by setting the search range within a range highly possiblyincluding the maximum power point over past experiences, a search atshort times is enabled. However, in the case of a solar light panel, thecurved line that indicates the current-voltage characteristic mayindicate multiple peak points. In this case, when the search range ispurposefully narrowed down, only the peak point within such a range issearched, enabling a search at short times. Conversely, when the searchrange is extended, although the search time becomes long, a possibilitythat a true maximum power point is searched increases. That is, settingof the search range has a strategic reason for up to which peak pointthe search will be made in relation to the search time.

As for the reason (2), according to the power converter 400, when asearch is made to a low voltage in the MPPT control, the searchapproaches the short-circuit current, causing a large current to flow.Because of the specification of the power converter 400, the currentthat can flow has a limit, and thus a search beyond this limit is notallowed in some cases. That is, the upper limit of the voltage or thecurrent and the lower limit thereof which enable the power converter 400to operate properly are always present. Hence, the upper limit of thesearch range and the lower limit thereof may be set in accordance withthe design of the power converter 400. That is, setting of the searchrange has a reason that the power converter 400 has an individualrestriction.

The search range according to this embodiment is not limited to anyparticular range, but is an appropriate range in consideration of theabove (1) and (2). As an example, the search range may be any one of thefollowings, or a combination of some of or all of the followings:fluctuation width by a temperature coefficient; the fluctuation widthper a product; a detection error; and a margin. The fluctuation width bya temperature coefficient is a variability of the maximum power pointdue to a temperature coefficient in a reference status (STC) definedbased on a measurement condition, such as the light intensity, thespectrum, or the temperature. The fluctuation width per a product is avariability caused product by product of the mass-produced power supply200. The detection error is an error in a detection value of a sensor,the control device 500, etc. The margin is a leeway range.

The start position instructing unit 36 is a process unit that starts,when the search by the searching unit 31 reaches the upper limit of thepreset search range, the next search from the lower limit of the searchrange, and starts, when the search by the searching unit 31 reaches thelower limit of the search range, the next search from the upper limit ofthe search range. The search terminating unit 37 is a process unit thatterminates the search when the search by the searching unit 31 reachesthe upper limit of the search range or the lower limit thereof by apredetermined number of times. This predetermined number of times is setin the memory 10 beforehand.

[Action]

An action according to this embodiment as explained above will beexplained with reference to FIG. 10. FIG. 10 illustrates the upper limitof the search range and the lower limit thereof in a voltage-powercharacteristic. For example, the power supply 200 remarkably changes thevoltage-power characteristic in accordance with a change in externalenvironment. For example, in the case of a solar battery, thevoltage-power curve changes in various forms in accordance with atemperature and a difference in illumination intensity among cells.Hence, an operation not intended is expected.

In this embodiment, as illustrated in FIG. 10, when the search by thesearching unit 31 reaches the lower limit of the search range, the startposition instructing unit 36 starts the searching operation from theupper limit. Conversely, when the search by the searching unit 31reaches the upper limit of the search range, the start positioninstructing unit 36 starts the search operation from the lower limit.Hence, the searching unit 31 searches the maximum point always withinthe range.

When the search by the searching unit 31 reaches the upper limit of thesearch range or the lower limit thereof by the predetermined number oftimes, the search terminating unit 37 terminates the search. That is,when the above operation is repeated by the predetermined number oftimes, the searching unit 31 terminates the search operation at theupper limit or the lower limit.

[Effects]

As explained above, the voltage-power characteristic is not like thecharacteristic illustrated in FIG. 10. However, according to the searchby the hill climbing method, a search may sometimes performed on a hilldifferent from the maximum point, resulting in a dead-end at the upperlimit of the search range in the generated power maximizing control andat the lower limit thereof. Hence, the search does not reach the maximumpoint of the generated power in some cases.

When, for example, the search reaches the upper limit or the lower limitalthough power is increasing during the search, the searching operationmay be stuck in this condition, and may become difficult to get out fromthis stuck condition. Such a case may be avoidable by a certainalgorithm.

According to this embodiment, however, such an algorithm is unnecessary,and when the search reaches the lower limit, the search is started againfrom the upper limit, and when the search reaches the upper limit, thesearch is started again from the lower limit. Such a simple processsuppresses a stuck at the upper limit and the lower limit, enabling asure search that reaches the maximum power point.

However, a stuck at the upper limit and the lower limit is not always atechnical problem, and may have a possibility that is a result of thesearch of the maximum power point. Hence, when the above operation isrepeated by the predetermined number of times, the search operation maybe terminated at the upper limit and at the lower limit.

Fifth Embodiment

[Configuration]

This embodiment employs basically the same configuration as that of thesecond embodiment. However, as illustrated in FIG. 11, the MPPT controlunit 30 of this embodiment includes a suppressing unit 38 thatsuppresses a rapid change in operation voltage or operation current. Forexample, this suppressing unit 38 is a process unit that gives aderivative limit to the instructed value of the voltage or current inthe generated power maximizing control. That is, the suppressing unit 38puts a slope limit that is dv/dt or di/dt in the instructed value tosuppress a rapid increase or a rapid decrease.

[Action]

An action of this embodiment as explained above will be explained withreference to FIG. 12. In this embodiment, the suppressing unit 38suppresses a rapid increase and a rapid decrease of the instructed valuefor the voltage or current relative to the converter control unit 20 bythe slope limit during the search by the searching unit 31. Hence, arapid increase and a rapid decrease of the voltage of a capacitor or ofthe current of a reactor which is the energy buffer of the powerconverter 400 is suppressed.

Since the control cycle is constant, for example, the more the voltagestep with is increased, the higher the possibility that the energykeenly flows in the energy buffer and the energy buffer breaks downincreases. Hence, the suppressing unit 38 suppresses a keen flow.

Although a time interval for changing the operation voltage or theoperation current remains the same, a rapid change in generated power issuppressed. Hence, there is no adverse effect to the search time. When alow-pass filter is applied as the suppressing unit 38, the similarprocess to the above explained process is accomplishable.

[Effects]

As explained above, according to this embodiment, even if the voltage ofthe capacitor or the current of the reactor which is the energy buffercannot be controlled at fast speed, the downsizing and cost reduction ofthe energy buffer are accomplishable. Consequently, the downsizing andcost reduction of the power conversion device 100 are accomplished.

Sixth Embodiment

[Configuration]

This embodiment employs basically the same configuration as the secondembodiment. However, as illustrated in FIG. 13, the MPPT control unit 30of this embodiment includes an average value calculating unit 39. Theaverage value calculating unit 39 is a process unit that obtains theaverage value of the voltage value or the current value within apredetermined range of the cycle of AC power as the operation voltage orthe operation current for the searching unit 31 that searches themaximum power point.

[Action]

The action of this embodiment explained above will be explained withreference to FIG. 14. First, as explained above, the searching unit 31checks the increase or decrease of the generated power by the hillclimbing method. In this case, there are a fluctuation in the voltage orthe current due to the switching operation of the switching elements inthe power converter 400 and a fluctuation when the output by the powerconversion device is AC. Hence, constant power is not always maintained,and there is a minor increase or decrease.

For example, such a pulsation component may be eliminated by adding acapacitor at the power-supply—200 side that is the input side of thepower converter 400. However, the pulsation component originating fromthe frequency of the output by the power converter 400 becomes arelatively low frequency, and it is difficult to completely eliminatesuch a pulsation component or a large capacitor should be added.

In this embodiment, the average value calculating unit 39 obtains theaverage value in such a way that the searching unit 31 does not falselydetermine the increase or decrease of the generated power due to thefluctuation in voltage or current. In this case, as for the AC powerpulsation, a double frequency and a sixfold frequency are dominant inthe case of the single phase system and the three phase system,respectively. Hence, when, for example, the average value calculatingunit 39 calculates the average power at a half cycle s of AC, thefluctuation becomes ignorable for the searching unit 31.

In addition, in present single-phase systems, the power to be controlledby the power converter may contain a first frequency, i.e., an ACfrequency due to an adverse effect of harmonic from a load like ahalf-wave rectifier that causes a current to flow only when the voltageis positive or negative. In this case, the average value calculatingunit 39 calculates the average power at a cycle S of AC. This enablesthe searching unit 31 to ignore the fluctuation.

[Effects]

As explained above, according to this embodiment, a false determinationon the increase and decrease of the generated power is preventablewithout a need of a large capacitor at the input side of the powerconverter 400. This enables downsizing, cost reduction and controlprecision improvement of the power conversion device 100.

Examples

Examples that show the effects of the above embodiments will beexplained with reference to FIGS. 15, 16, and 17. In FIGS. 15, 16, and17, respective horizontal axes indicate a time [s], respective verticalaxes at the upper stage indicate a voltage instruction [V], respectivevertical axes at the middle stage indicate a solar battery current [A]as a power supply, and respective vertical axes of the lower stageindicate generated power [W] per a time. The side blow-off in eachfigure is an enlarged diagram for a part of the upper, middle, and lowerstage.

In addition, FIG. 15 is a comparison example when the voltage step widthwas set to a relatively large constant value, FIG. 16 is a comparisonexample when the voltage step width was set to a relatively smallconstant value, and FIG. 17 is an example when the voltage step widthwas changed like the second embodiment.

According to FIG. 15, in the generated power maximizing control, sincethe set voltage width was large, a necessary time after the start of thecontrol and until the maximum power point was found was short. Since thevoltage step width was large, however, there was a step at which thegenerated power temporarily decreased. Accordingly, although a time tofind the maximum power point is short, the amount of generated powerdecreases.

According to FIG. 16, in the generated power maximizing control, sincethe set voltage step was small, there was little step at which thegenerated power temporarily decreased, but a necessary time to find themaximum power point was long. Accordingly, there was a delay in findingthe maximum power point, and it was difficult to cope with atime-by-time change in solar light emitted to solar batteries. Thisresulted in a loss of power generation opportunity, thereby decreasingthe amount of generated power.

FIG. 17 illustrates the example of the embodiment, and shows a waveformwhen the voltage step was changed. A necessary time to find the maximumpower point was short, and from this condition by decreasing the voltagestep, an occasion in which the generated power temporarily decreased wasavoided. Therefore, the maximum power point was found at fast speed,while at the same time, the amount of generated power was large.

Note that the example in FIG. 17 is a circuit analysis result to showthe operation of decreasing the voltage step. However, as explained inthe first embodiment, the operation of increasing the voltage step isalso applicable. Since both operations are similar to each other, thedetailed explanation will be omitted. In addition, an amount of changeto decrease or increase the voltage step is not limited to anyparticular amount. That is, according to the operation of decreasing thevoltage step, the voltage may be changed like 4 V, 3 V, 2 V, and 1 V inthis order in sequence. Conversely, according to the operation ofincreasing the voltage step, the voltage may be changed like 1 V, 3 V,and 4 V in this order in sequence.

Other Embodiments

(1) The above embodiments are not limited to the above examples. Forexample, various power converters for the power conversion device andvarious power supplies to be connected thereto are applicable. Hence,the power conversion device is not limited to the so-called PCS only,and may be any devices which are capable of performing the MPPT controland the converter control. In addition, the power conversion device maybe a micro-inverter that is connected to each of a large number of solarbattery modules.

(2) The specific details, values, etc., of the information applied tothe above embodiments are optional, and are not limited to specificdetails and values. For example, the preferable setting value set up inmemory vary depending on the power generation status of the powersupply, the specification of the power supply and that of the powerconverter, etc. In addition, the setting value may be dynamicallychanged in accordance with the operation status of the power supply andthat of the power converter. Still further, in the large-smalldetermination, the matching-mismatching determination, etc., relative tothe setting value according to the above embodiments, the determinationmay be made based on criteria like equal to or greater than and equal toor smaller than so as to include the value subjected to determination,or on criteria like greater than, smaller than, and lower than so as toexclude such a value.

Several embodiments of the present disclosure have been explained above,but those embodiments are merely presented as examples, and are notintended to limit the scope of the present disclosure. Those embodimentscan be carried out in other various forms, and various omissions,replacements, and modifications can be made thereto without departingfrom the scope of the present disclosure. Such embodiments and modifiedforms are within the scope of the present disclosure, and are alsowithin the scope of the invention as recited in the appended claims andthe equivalent range thereto.

REFERENCE SIGNS LIST

-   -   100 Power supply system    -   200 Power supply    -   300 PCS    -   400 Power converter    -   500 Control device    -   10 Memory    -   20 Converter control unit    -   30 MPPT control unit    -   31 Searching unit    -   32 Consecutive time determining unit    -   33 Step width increasing unit    -   34 Repeat time determining unit    -   35 Step width decreasing unit    -   36 Start position instructing unit    -   37 Search terminating unit    -   38 Suppressing unit    -   39 Average value calculating unit

The invention claimed is:
 1. A control device for a power converter, thecontrol device comprising: a searching unit increasing or decreasing, ata predetermined step width, an operation voltage or an operation currentof a power supply connected to the power converter to search a maximumpower point of the power supply; a consecution time determining unitdetermining whether or not the searching unit has consecutivelyincreased or decreased the operation voltage of the power supply or theoperation current thereof; and a step width increasing unit increasingthe step width upon determination by the consecution time determiningunit that the increase or the decrease has been consecutively executedby a predetermined number of times.
 2. The power-converter controldevice according to claim 1, wherein the searching unit comprises: arepeat time determining unit determining whether or not the increase orthe decrease of the operation voltage of the power supply or theoperation current thereof has been consecutively repeated by thepredetermined number of times; and a step width decreasing unitdecreasing the step width upon determination by the repeat timedetermining unit that the repeat has been consecutively made by thepredetermined number of times.
 3. The power-converter control deviceaccording to claim 1, further comprising a step width setting unitsetting a minimum width of the step with based on a resolution of acarrier wave for a PWM control on the power converter.
 4. Thepower-converter control device according to claim 1, further comprisinga start position instructing unit starting, when the search by thesearching unit reaches an upper limit of a preset search range, a nextsearch from a lower limit of the search range, and starting, when thesearch by the searching unit reaches the lower limit of the searchrange, the next search from the upper limit of the search range.
 5. Thepower-converter control device according to claim 4, further comprisinga search terminating unit terminating the search when the search by thesearching unit has reached the upper limit of the search range or thelower limit thereof by a predetermined number of times.
 6. Thepower-converter control device according to claim 1, further comprisinga suppressing unit suppressing a rapid change in the operation voltageor the operation current.
 7. The power-converter control deviceaccording to claim 6, wherein the suppressing unit comprises a low-passfilter causing the operation voltage or the operation current at aninstructed value to pass.
 8. The power-converter control deviceaccording to claim 6, wherein the suppressing unit gives a derivativelimit to an instructed value for the operation voltage or the operationcurrent.
 9. The power-converter control device according to claim 1,wherein: the power converter is a converter converting DC power into ACpower of a predetermined cycle; and the control device further comprisesan average value calculating unit obtaining an average value of avoltage value or a current value within a predetermined range of thecycle of the AC power as an operation voltage or an operation currentfor the searching unit to search the maximum power point.
 10. Thepower-converter control device according to claim 1, wherein a startposition of the search by the searching unit is set to a positionequivalent to a short-circuit current or an open voltage.
 11. Thepower-converter control device according to claim 1, wherein a startposition of the search by the searching unit is set to a high-voltageside relative to the maximum power point.
 12. A power-converter controlprogram causing a computer to execute: a searching process of increasingor decreasing, at a predetermined step width, an operation voltage or anoperation current of a solar battery connected to a power converter tosearch a maximum power point of the solar battery; a consecution timedetermining process of determining whether or not, through the searchingprocess, the operation voltage of the solar battery or the operationcurrent thereof has been consecutively increased or decreased by apredetermined number of times; and a step width increasing process ofincreasing the step width upon determination through the consecutiontime determining process that the increase or the decrease has beenconsecutively executed by a predetermined number of times.
 13. A powerconversion device comprising: a power converter; a converter controlunit controlling the power converter; and the control device accordingto claim 1.